Binoculars are first and foremost characterized by a pair of numbers like 8 × 30 or 7 × 50. The first number is the magnification and the second shows the aperture in millimeters. For astronomy, the aperture should be as large as possible and not less than 30mm. Smaller devices don‘t gather enough light and often they are of low quality – many are cheap toys and not optical devices. With 30mm aperture you can already see much of the sky and 50mm are quite powerful – many a department store telescope has a similar aperture. Larger models are quite heavy and unwieldy, so they are especially useful for purely astronomical purposes. Because the price also rises rapidly with the aperture, they are more suitable for the ambitious user. By the way, the increase in light-gathering between naked eye and 50mm binoculars is similar to the gain when switching from binoculars to a telescope with about a 14” aperture – which costs several thousand dollars instead of less than 200 dollars.
The magnification influences not only the field of view, but also the image brightness and contrast. At low magnification, the field of view is usually larger, and the binocular is perfectly suited to observe star clusters or the Milky Way. With increasing magnification more details become visible, but the image gets darker. This is not necessarily a disadvantage – if you are under a bright, light-polluted sky, a slightly higher magnification helps to darken the sky background and thus to enhance the contrast. The background brightness is thereby evenly distributed over a larger area in the eye, while the point-like stars continue to be perceived only as points.
If you are observing the sky with a 50mm model mostly in the vicinity of urban areas, you‘ll probably see more at 10x magnification than with a 7x magnification. Binoculars with seven times magnification are ideal especially for star clusters and the Milky Way, while at ten times magnification or more somewhat fainter deep-sky objects become visible. Large binoculars from about fifteen times magnification are even suitable for observing fine details on smaller objects or the Moon and planets.
The 7 × 50 can be seen as a typical „all-purpose binocular“. But often, a 10 × 50 is the better choice since we never reach the perfect dark adaptation in North America or close to cities.
An important number which can be calculated easily is the exit pupil. It is obtained by dividing the aperture by the magnification. A pair of 7 × 50 binoculars for example has an exit pupil of 50mm / 7 = 7.14mm, and with a 10 × 50 this value is reduced to 5mm. The exit pupil tells you how large the beam of light is when it leaves the eyepiece. It should not be larger than the opening of your pupil, but also not significantly smaller. As a rule of thumb, young people usually have a maximum pupil aperture of 7mm, and at the age of 30 it drops to 5mm – but these are only averages. At the age of 25, for example, openings from 4.5 to 8.5mm were measured, and values of 3 to 6.5mm for 55 year old people. The individual values are influenced by exercise and training as well as by genetics. They are also affected by the ambient brightness – if you observe in light-polluted, urban places, you‘ll never observe with maximum pupil opening.